MEASUREMENT
7. Pumping Speed
5. Pressure Gauges
5.1 Direct –Reading Gauges
5.2 Indirect –Reading Gauges
6. Flow Meters
6.1 Molar Flow, Mass Flow, and
Throughput
6.2 Rotameters and Chokes
6.3 Differential Pressure Techniques
6.4 Thermal Mass Flow Meter
Techniques
7.1 Pumping Speed
7.2 Mechanical Pumps
7.3 High Vacuum Pumps
8. Residual Gas Analyzer
8.1 Instrument Description
8.2 Installation and Operation
8.3 RGA Calibration
8.4 RGA Selection
9.Interpretation of RGA Data
9.1 Cracking Patterns
9.2 QualitatieAnalysis
9.3 Quantitative Analysis
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Chapter 5 Pressure Gauges
Direct gauge: By calculating the
force exerted on the surface by
incident particle flux.
Indirect gauge: By measuring a
gas property that changes in a
predictable manner with gas density.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
3.3 Pressure Measurement
3.3.1 Mechanical Gauges –Liquid walls, direct gauge
Mercury and oil manometer
** Hg manometer useful down to 1 torr.
** Oil manometer useful down to 0.1 torr.
** Avoid the capillary effect by oil.
** Remove gases inside oil.
** Avoid chemical reaction of Hg with
gases by floating a few drops of silicone
oil on top of the Hg column.
** Vapor pressure of Hg and oil can be
removed by placing a cold trap.
Note: Are you measuring a relative or absolute pressure?
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Hg at 20°C
Density = 13.54562 g/cm3
Pvapor = 0.0012 mmHg
3.3.1 Mechanical Gauges –Liquid walls, direct gauge
McLeod Gauge:
P1V1 = P2V2 = (ρgh)σh　 P1 = ρσgh2/ V1
** A sample of gas is trapped and compressed by a known amount
(~ 1: 1000).
** The pressure of the compressed gas is measured with a mercury manometer,
while the original pressure is determined by the gas law.
** This method is not applicable to a gas that condenses upon compression.
** The McLeod gauge is limited by geometrical constraints to about four orders of
magnitude in pressure.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.1. DIRECT-READING GAUGES
5.1.1 Diaphragm and Bourdon
Metallic diaphragm gauge
- Rough indication of pressure
** The volume on one side of the diaphragm is sealed.
** The volume on the other side is attached to the system.
** A variation in pressure on one side relative to the other causes the diaphragm to
flex.
-- The precision of both solid-walls gauges are limited by hysteresis caused by
friction in the linkage.  Tap the gauge before use.
-- The solid-walls gauges are not absolute, they have to be calibrated against an
absolute gauge (i. e. McLeod)
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.1.1 Diaphragm and Bourdon
Bourdon gauge
Rough indication of pressure
** A thin-walled metal tube, somewhat flattened and bent into the form of a C.
** Attached to its free end is a lever system that magnifies any motion of
the free end of the tube.
** On the fixed end of the gauge is a fitting to the measured system.
** As pressure increases within the measured system, it travels through the tube.
-- Like the snakelike paper whistle,
the metal tube begins to straighten as the pressure increases inside of it.
** As the tube straightens, the pointer moves around a dial that indicates the
pressure typically in psi (14.7 psi= 1 atm).
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.1.1 Diaphragm and Bourdon
Bourdon gauge
-- Rough indication of pressure
Compound gauge: to
measure both vacuum and
pressure.
Differential gauge: to
measure the pressure
difference.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.1.1 Diaphragm and Bourdon
** Pressure = Force /area
-- 1 atm = 0 psig = 14.7 psia
** Vacuum – The removal of gas molecules in a close
container to achieve a pressure less than atmosphere.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.1.2 Capacitance Manometers
** A capacitance gauge is simply a diaphragm gauge.
** The deflection of the diaphragm is measured by observing the change in
capacitance between it and a fixed counter electrode.
** Measuring absolute pressures.
** The diaphragm provides very low hysteresis, excellent repeatability, remarkably
high resolution (1x10-5 of Full Scale), fast response, and the ability to measure
extremely low pressures
.** A transducer and an electronic sense unit
convert the membrane position to a signal
linearly proportional to the pressure.
Double-sided capacitance manometer:
** 1% difference in dielectric constant will
result in a 0.5% error. (Disadvantage)
-- Null detector
-- Direct reading gauge (Pref < 10-5 Pa)
Reference
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.1.2 Capacitance Manometers
Single-sided capacitance manometer
** The high vacuum on the reference side is maintained over the life of the
manometer by means of an internal chemical getter pump.
** Deflection can be as low as 10-9 cm, motion of parts due to temperature change
becomes a large source of error.  Ambient temperature at ~ 50ºC.
-- Without proper temperature regulation, errors of (1) zero and span coefficient of
5 –50 ppm full scale, and (2) 0.004 –0.04% of reading per degree Celsius.*
** Absolute pressure operated over a large dynamic range.
0.1 torr–10,000 torr.
Similar to Fig. 5.4 (P. 85)
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Piezo-diaphragm vacuum gauges (Micro-Machined Silicon pressure transducers)
Pressure-induced strain increases the value of the radial resistors "r", and
decreases the value of resistors "t" tangential to the radius. This resistance-change
can be as high as 30%. The resistors are hooked up as a Wheatstone bridge. The
bridge output is directly proportional to the pressure.
http://fieldbus.feld.cvut.cz/en/system/files/files/en/education/courses/xe38ssd/05-Pressure.pdf
Grace H. Ho, Department of Applied Chemistry, National University
of Kaohsiung
http://medhycos.mpl.ird.fr/en/data/tec/sensors/keller1bis.pdf
5.2 INDIRECT-READING GAUGES
** By measuring a pressure-dependent property of the gas.
** P > 0.1 Pa in the medium vacuum range region.
-- Energy transfer technique, thermal conductivity: Pirani or thermocouple gauges .
-- Momentum transfer technique, viscosity: Spinning rotor gauge.
** 0.1 -10-10 Pa in the high to ultrahigh vacuum region, measuring gas density.
-- Ionization gauges:
Bayard –Alpert and extractor hot cathode gauges,
Penning ion gauge, cold cathode gauges.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.2.1 Thermal Conductivity Gauges
** Thermal conductivity = constant
P > 1 torr　 0
P ~ 10-3 torr
** Knudsen number, Kn = mean free path / system dimension = λ/ d
Viscous flow, Kn < 0.01
Free molecular heat flow  P, 0.01 < Kn < 10
** Used as only rough indicators of vacuum.
Convectron gauge
A fewλ~ d
λ~ d
Linear region
Thermalcouple, Pirani gauge
Thermal energy is transferred by gas
bouncing between the heat source and
the chamber case.
** To extend the range of a gauge to its lowest
possible pressure limit:
Heat flow losses ∝ (T24 – T14) + end losses
-- End losses dominates when the wire is short.
-- Radiation losses increase with wire temperature.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.2.1 Thermal Conductivity Gauges
Pirani gauge
A heated wire
forms one arm of
a Wheatstone
bridge.
** A Wheatstone bridge circuit is used to measure the resistance change.
** Nulling: The gauge tube is evacuated to ~ 10-4 Pa and R1is adjusted for balance.
** Compensating tube: To correct for temperature induced changes on nulling.
With the compensating (evacuated and sealed) tube, pressure can be measured to
10-3 Pa.
** A low-temperature filament is heated by a constant temperature, current, or
voltage.
-- Constant temperature: The most sensitive and accurate method. As pressure in
the gauge tube increase, more heat is dissipated.  Increasing the voltage to move the bridge toward balance.
-- Constant voltage or current: The out-of-balance current meter is simply
read Chemistry,
the pressure.
Grace H.calibrated
Ho, Departmentto
of Applied
National University of Kaohsiung
5.2.1 Thermal Conductivity Gauges
** Constant current is delivered to the heated wire.
** Temperature of the filament depends on the rate of heat loss to the surrounding
gas. (Thus, its working range is about 1 – 10-3 torr.)
**
TC gauge: The temperature is determined by the e.m.f. produced by a
Chromel-Alumel thermocouple (K-type) in contact with the filament.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.2.1 Thermal Conductivity Gauges
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.2.1 Thermal Conductivity Gauges
** The pressure indicated by the thermal-conductivity gauges depends upon the
thermal conductivity of the gas.
** The pressure readings are calibrated by the manufacturer for use with air.
** These gauges can only be expected to be accurate within a factor of two.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
3.3.2 Thermal-Conducting Gauges – Convectron, indirect
** Convectron gauges provide useful pressure measurement over 6 decades from 1
millitorr to 1000 Torr, or from 1 x 10-3 mbar to 1000 mbar.
** This vacuum measurement is accomplished with a single gauge tube.
** The gauge tube contains a temperature compensated heat loss sensor which
utilizes conduction cooling to sense pressure at lower pressures. (10-3 to 1 torr)
** At higher pressures, it utilizes convection cooling in which gas molecules are
circulated through the gauge tube by gravitational force.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
3.3.2 Thermal-Conducting Gauges – Convectron, indirect
** P < 1 torr, not sensitive
to the gas identity.
** P > 1 torr, very sensitive
to the gas identity.
e. g. Ar at 1000 torr can be
read as at “30 torr” by
the device!
 Dangerous!!
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.2.3 Ionization Gauges – Hot Cathode Gauge
** Usable in the region of molecular flow. 10-3 – 10-10 torr.
** Electron impact ionization, the number of positive ions is proportional to the
number density, not the pressure. The current is also depends on the
ionization efficiency of the gas in the gauge. (Sensitivity factor)
** Electrons are accelerated through the gas toward a positively biased grid.
** Ions are collected by the ion collector (a fine wire to minimize interception of x-rays).
** The most common gauge is the thermionic or hot-cathode ionization gauge, the
most familiar configuration is devised by Bayard and Alpert (B-A type).
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.2.3 Ionization Gauges – Hot Cathode Gauge
Limitation to the low end of the operation range: X-ray
** The X ray limit varies with different gauge designs.
** X rays are produced when the electrons emitted by the cathode impact the grid.
** The B-A type ion gauge means to minimized the
x-ray effect.
** When the x-rays strike the collector to produce
photoelectrons, the photoelectron currents from
the current collectors is detected the same as
positive ions arriving at the ion collector.
Degassing
** An unbake tubulated gauge should be outgassed. An initial outgassing is typically
15 –20 min. Subsequently outgassing needs only ~ 15 sec.
** Outgassing at P < 10-4 Pa.
Ion gauge as a pump
** Ions accelerated to and imbedded in the collector  removed from the system.
** Metal evaporated from the filament deposits on the walls to produce a clean,
chemically active surface that adsorbs N2, O2, and H2O.
 Pumping speed is ~ 0.2 liter s-1 for N2 (@ electron-emission current of ~ 10 mA).
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.2.3 Ionization Gauges – Hot Cathode Gauge
“True pressure”
= Pressure reading / relative sensitivity
Px  P( meterreading ) CFx
**
Sensitivity in units of microamperes of collector
current per unit of pressure
per manufacturer’s specified emission current
e. g.
N2: (100 μA / mTorr)/ 10 mA
Filaments:
-- Commonly used materials for filaments: tungsten or thoriated iridium.
-- More burnout resistant: Coated iridium filaments. Tungsten filaments burn out
immediately when exposed to pressures of 0.01 torr or higher .
-- More chemical resistant: Reactive tungsten filaments, especially to halogens.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.2.3 Ionization Gauges – Hot Cathode Gauge
“True pressure” = Pressure reading / relative sensitivity
Px  P( meterreading ) CFx
MKS
SRS (Standford
research systems)
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.2.3 Ionization Gauges – Hot Cathode Gauge
Hot cathode gauge errors:
-- X-ray generated photocurrent: fine wire for the ion-collector.
-- Electron – stimulated desorption
Gas could not adsorb on
-- Wall outgassing
grids at high temperature.
(Why IG operated at 10 mA.)
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Outdiffusion
of H2 from
the grid wire.
5.2.3 Ionization Gauges – Cold Cathode (Penning) Gauge
Electrons go back and forth through the anode,
and finally collide with the anode.
S
2 to 10 kV
N
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
5.2.3 Ionization Gauges – Cold Cathode (Penning) Gauge
ULVAC
Thermionics
Cold-cathode ionization gauges
** The cold cathode ionization gauge ionizes the residual gas by an electric discharge.
** A magnetic field forces the electrons on a longer distance in spiral paths before
finally colliding with the anode, thereby increasing the ionization probability.
** Cold cathode ionization gauges are rugged and simple. Not mounted immediately
adjacent to a hot cathode gauge or residual gas analyzer.
** No X-ray limit, little ESD or thermally induced wall outgassing.
** These gauges are used in the vacuum range from 1 – 10-9 Pa (10-11 torr), therefore,
they may have difficulties starting a discharge at very low pressure unless the
gauge contains an auxiliary source. (Starting parameter ~ 50 – 500 uPa-s, w/0 Aux.)
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Semiconductor Requirement
The TripleGauge (Inficon) consists of
a Bayard-Alpert hot ionization gauge,
a Pirani thermal-conductivity gauge,
and a miniature alumina capacitance
diaphragm gauge. This sensor is
capable of covering vacuum
measurements from 5×10-10 to 1500
mbar in a single housing.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Semiconductor Requirement
http://www.mksinst.com/docs/UR/900seriesDS.pdf
The transducers incorporate MEMS (Micro Electro-Mechanical Systems) based
technologies, including MicroPirani™ and Piezo sensors, combined with both Cold
Cathode and Mini Ion BA technology resulting in a broad product offering for a wide
variety of customer applications.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung
Summary of
Pressure Gauges
Gauge calibration:
-- Direct comparison
-- Series expansion
By government
standards institutions
and gauge
manufacturers.
Grace H. Ho, Department of Applied Chemistry, National University of Kaohsiung